Butanol is a compound which is industrially very important as a raw material of chemicals and pharmaceuticals, and as a solvent and a fuel. Butanol is generally produced by a chemical synthesis method using propylene as a raw material (oxo method), but, in view of the recent problems of decrease in the crude oil resource and sharp rise in its prices, and GHG (greenhouse gas) emission control, technologies for butanol production by microbial fermentation using biomass, which is non-fossil material, as raw material have been drawing attention, and several such technologies have been reported (e.g., Patent Document 1). However, in general, production of butanol by microbial fermentation allows accumulation of butanol in the fermentation broth up to only about 1 to 3% by weight, because of inhibition of the growth of the microorganism by butanol. Therefore, in order to obtain pure butanol from the fermentation broth, a large quantity of water contained in the fermentation broth needs to be removed. As a common method for removal of water, vacuum heating is employed, but removal of water is difficult because butanol has a property to easily undergo azeotropy with water.
In Patent Document 2, as a method for separating/purifying butanol from a butanol-containing solution produced by microbial fermentation, a method is disclosed wherein a fermentation broth is concentrated with a reverse osmosis membrane and the butanol phase of the resulting two-phase-separated concentrate is subjected to distillation to recover butanol. However, fermentation broths generally contain impurities such as inorganic salts, saccharides and proteins derived from fermentation media; and alcohols and organic acids generated as side products. These impurities easily cause fouling of the membrane and the osmotic pressure may be increased thereby, resulting in requirement of a higher pressure to achieve concentration of the broth to a level at which two-phase separation occurs. Further, in cases where an impurity having surface activity is contained, two-phase separation is less likely to occur, which is problematic. Still further, since impurities such as colored components are contained in the butanol phase, it may be difficult to purify, by distillation, butanol at high purity with low degree of coloration, which is problematic. Since Patent Document 2 describes neither examples using a reverse osmosis membrane nor effects of impurities in cases of concentration through a reverse osmosis membrane, it has not been clear whether the method described in Patent Document 2 can be applied to production of butanol.
Further, Patent Document 3 discloses a method for recovering a solvent contained in a dilute aqueous solution with a nanofiltration membrane, and butanol is included as a specific example of the solvent. This method aims to recover a solvent such as butanol from the feed side of the nanofiltration membrane, and the mechanism of recovery of butanol in this method suggests to those skilled in the art that a nanofiltration membrane is not permeable to butanol. Patent Document 3 does not disclose any actual example of filtration of a butanol-containing solution through a nanofiltration membrane, and there is no description on the two-phase separability of the recovered aqueous butanol solution.